Parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP) play important physiological roles in calcium homeostasis and in development, respectively. Calcium concentration in the blood is tightly regulated, due to the essential role of calcium in cell metabolism. PTH is an endocrine hormone which is secreted from the parathyroid gland in response to decreased serum calcium levels. PTH acts directly to increase bone resorption and to stimulate renal calcium reabsorption, thus increasing or preserving circulating calcium stores. PTH also indirectly increases calcium absorption in the gut by stimulating the renal hydroxylation of vitamin D.
Both primary and secondary hyperparathyroidism are conditions that are associated with excessive levels of circulating parathyroid hormone. Through the aforementioned pathways, excess PTH levels can cause hypercalcemia and osteopenia. Bone resorption inhibitors such as bisphosphonates and OPG can effectively protect bone and can inhibit the skeleton""s contribution to hypercalcemia. However, the calcemic effects of hyperparathyroidism on the kidney and gut are not addressed by currently available therapy.
PTHrP is produced by many cell types, and plays an important role in regulating skeletal development. Postnatally, the roles for PTHrP are less clearly defined. Circulating levels of PTHrP are essentially non-detectable in normal healthy adults. However, many tumors of diverse embryological origins produce and secrete PTHrP in quantities sufficient to cause hypercalcemia. In fact, humoral hypercalcemia of malignancy (HHM) is the most common paraneoplastic syndrome, which accounts for significant patient morbidity and mortality.
Currently, HHM is treated with saline hydration followed by bone resorption inhibitors such as bisphosphonates. This treatment regimen typically takes 3-4 days to achieve significant reductions in serum calcium, and the effects are relatively short-lived (less than one month). For patients with high circulating levels of PTHrP, the effects of current treatment options are even less impressive. Repeated administration of conventional therapies are usually progressively less effective. These limitations to current therapy strongly indicate an unmet medical need for rapid, effective, and long-lasting treatments for HHM.
A major reason for the limited benefits of current HHM therapy is the failure to directly inhibit PTHrP, which is very well established as the principal pathophysiologic factor in HHM. Bone resorption inhibitors such as bisphosphonates only inhibit bone resorption, while PTHrP also has significant calcemic effects on the kidney and the gut. Total neutralization of PTHrP would be the ideal adjuvant therapeutic approach to treatment of HHM.
Both PTH and PTHrP interact with PTH-1 receptor, which accounts for most of their known effects. Mannstadt et al. (1999), Am. T. Physiol. 277. 5Pt 2. F665-75 (1999). Only PTH interacts with the newly discovered PTH-2 receptor. Id. PTHrP can be changed to a PTH-2 receptor agonist, however, by changing two residues to the residues at those positions in PTH. Gardella et al. (1996), J. Biol. Chem. 271 (33): 19888-93.
An N-terminal fragment of PTH has been used as a therapeutic agent. Intermittently administered native PTH-(1-84) exhibits osteogenic properties, and it has been recognized for decades that these properties can be fully realized with the C-terminally truncated fragment PTH-(1-34). Both peptides bind and activate the PTH-1 receptor with similar affinities, causing the activation of adenylate cyclase (AC) as well as phospholipase C (PLC). AC activation through PTH-1 receptor generates cAMP, while PLC activation through PTH-1 receptor generates PKC and intracellular calcium transients. PTH-(1-34) can maximally activate both the AC and the PLC pathways. It has been demonstrated that the anabolic effects of PTH-(1-34) require short intermittent (daily) exposures Dobnig (1998), Endocrinol. 138: 4607-12. In human trials on postmenopausal women, daily subcutaneous injection of low doses of PTH(1-34) were shown to result in impressive bone formation in the spine and femoral neck with significant reduction in incidence of vertebral fractures. These clinical data reveal PTH as one of the most efficacious agents tested for osteoporosis.
Truncated PTH fragments have diminished AC/cAMP activation and similarly diminished anabolic activity. Rixon et al. (1994), J. Bone Min. Res. 9: 1179-89; Hilliker et al. (1996), Bone 19: 469-477; Lane et al. (1996), J. Bone Min. Res. 11: 614-25. Such truncated PTH fragments have this diminished activity (Rixon et al. (1994); Hilliker et al. (1996); Lane et al. (1996)) even if they maintain full agonism towards PKC. Rixon et al., (1994). These observations have led to the proposal that the AC/cAMP pathway is critical for the bone anabolic properties of PTH, while the PLC/PKC pathway is dispensable in this regard. Rixon et al, (1994); Whitfield et al. (1996), Calcified Tissue International 53: 81-7.
An opposing, but not mutually exclusive, theory suggests that PLC activation (in addition to AC) might also be an important property of anabolic PTH fragments. Takasu (1998), Endocrinol. 139: 4293-9. The apparent absence of PLC activation by some anabolic C-terminally truncated PTH peptides may be an artifact of insensitive assay methods combined with lower receptor binding. Takasu (1998). Progressive truncations from the C-terminus of PTH-(1-34) result in stepwise reductions in binding affinity for the PTH1R Takasu (1998). PKC activation through PTH-1 receptor appears to be acutely sensitive to binding affinity and to receptor density (Guo et al. (1995), Endocrinol 136: 3884-91), whereas cAMP activation is far less sensitive to these variables. As such, hPTH-(1-31) has a slightly reduced (1-6 fold) affinity for PTH-1 receptor compared to hPTH-(1-34), while hPTH-(1-30) has a significantly reduced (10-100 fold) affinity Takasu (1998). Perhaps due to this decreased PTH-1 receptor affinity, PTH-(1-30) is a weak and incomplete agonist for PLC activation via the rat PTH-1 receptor.
Compared to PTH-(1-34), PTH-(1-31) has similar or slightly reduced anabolic potential (Rixon et al. (1994); Whitfield et al. (1996), Calcified Tissue International 53: 81-7; Whitfield et al. (1996), Calcified Tissue International 65:143-7), binding affinity for PTH1R, and cAMP induction (Takasu (1998)). PTH-(1-31) also has slightly reduced PLC activation. Takasu (1998). In healthy humans, infusion of PTH-(1-31) and PTH-(1-34) had similar stimulatory effects on plasma and urinary CAMP concentration, but unlike PTH-(1-34), PTH-(1-31) failed to elevate serum calcium, plasma 1,25(OH)2D3, or urinary N-TX levels. Fraher et al. (1999), J. Clin. Endocrin. Met. 84: 2739-43. These data suggest that PTH-(1-31) has diminished capacity to induce bone resorption and to stimulation vitamin D synthesis, which is a favorable profile for bone anabolic agents.
PTH-(1-30) was initially shown to lack anabolic properties Whitfield et al. (1996), Calcified Tissue International 53: 81-7. More recently, however, it has been demonstrated that PTH-(1-30) is anabolic when administered at very high doses (400-2,000 xcexcg/kg, vs. 80 xcexcg/kg for PTH-(1-34)). The lower potency of PTH-(1-30) could be predicted by its lower binding affinity for PTH-1 receptor, its diminished CAMP activation, and/or to its greatly diminished PKC activation. Takasu (1998). It remains to be determined whether PTH-(1-30) has a similar or even more desirable reduction in apparent bone resorption activity.
PTH-(1-28) is the smallest reported fragment to fully activate CAMP. Neugebauer et al. (1995), Biochem. 34: 8835-42. However, hPTH-(1-28) was initially reported to have no osteogenic effects in OVX rats. Miller et al. (1997), J. Bone Min. Res. 12: S320 (Abstract). Recently, a very high dose of PTH-(1-28) (1,000 xcexcg/kg/day) was shown to be anabolic in OVX rats, whereas 200 xcexcg/kg/day was ineffective. Whitfield et al. (2000), J. Bone Min. Res. 15: 964-70. The diminished or absent anabolic effects of some truncated PTH fragments has been attributed to rapid clearance in vivo. Rixon et al. (1994).
Recombinant and modified proteins are an emerging class of therapeutic agents. Useful modifications of protein therapeutic agents include combination with the xe2x80x9cFcxe2x80x9d domain of an antibody and linkage to polymers such as polyethylene glycol (PEG) and dextran. Such modifications are discussed in detail in a patent application entitled, xe2x80x9cModified Peptides as Therapeutic Agents,xe2x80x9d U.S. Ser. No. 09/428,082, PCT appl. no. WO 99/25044, which is hereby incorporated by reference in its entirety.
A much different approach to development of therapeutic agents is peptide library screening. The interaction of a protein ligand with its receptor often takes place at a relatively large interface. However, as demonstrated for human growth hormone and its receptor, only a few key residues at the interface contribute to most of the binding energy. Clackson et al. (1995), Science 267: 383-6. The bulk of the protein ligand merely displays the binding epitopes in the right topology or serves functions unrelated to binding. Thus, molecules of only xe2x80x9cpeptidexe2x80x9d length (2 to 40 amino acids) can bind to the receptor protein of a given large protein ligand. Such peptides may mimic the bioactivity of the large protein ligand (xe2x80x9cpeptide agonistsxe2x80x9d) or, through competitive binding, inhibit the bioactivity of the large protein ligand (xe2x80x9cpeptide antagonistsxe2x80x9d).
Phage display peptide libraries have emerged as a powerful method in identifying such peptide agonists and antagonists. See, for example, Scott et al. (1990), Science 249: 386; Devlin et al. (1990), Science 249: 404; U.S. Pat. No. 5,223,409, issued Jun. 29, 1993; U.S. Pat. No. 5,733,731, issued Mar. 31, 1998; U.S. Pat. No. 5,498,530, issued Mar. 12, 1996; U.S. Pat. No. 5,432,018, issued Jul. 11, 1995; U.S. Pat. No. 5,338,665, issued Aug. 16, 1994; U.S. Pat. No. 5,922,545, issued Jul. 13, 1999; WO 96/40987, published Dec. 19, 1996; and WO 98/15833, published Apr. 16, 1998 (each of which is incorporated by reference in its entirety). In such libraries, random peptide sequences are displayed by fusion with coat proteins of filamentous phage. Typically, the displayed peptides are affinity-eluted against an antibody-immobilized extracellular domain of a receptor. The retained phages may be enriched by successive rounds of affinity purification and repropagation. The best binding peptides may be sequenced to identify key residues within one or more structurally related families of peptides. See, e.g., Cwirla et al. (1997), Science 276: 1696-9, in which two distinct families were identified. The peptide sequences may also suggest which residues may be safely replaced by alanine scanning or by mutagenesis at the DNA level. Mutagenesis libraries may be created and screened to further optimize the sequence of the best binders. Lowman (1997), Ann. Rev. Biophys. Biomol. Struct. 26: 401-24.
Structural analysis of proteinxe2x80x94protein interaction may also be used to suggest peptides that mimic the binding activity of large protein ligands. In such an analysis, the crystal structure may suggest the identity and relative orientation of critical residues of the large protein ligand, from which a peptide may be designed. See, e.g., Takasaki et al. (1997), Nature Biotech. 15: 1266-70. These analytical methods may also be used to investigate the interaction between a receptor protein and peptides selected by phage display, which may suggest further modification of the peptides to increase binding affinity.
Other methods compete with phage display in peptide research. A peptide library can be fused to the carboxyl terminus of the lac repressor and expressed in E. coli. Another E. coli-based method allows display on the cell""s outer membrane by fusion with a peptidoglycan-associated lipoprotein (PAL). Hereinafter, these and related methods are collectively referred to as xe2x80x9cE. coli display.xe2x80x9d In another method, translation of random RNA is halted prior to ribosome release, resulting in a library of polypeptides with their associated RNA still attached. Hereinafter, this and related methods are collectively referred to as xe2x80x9cribosome display.xe2x80x9d Other methods employ peptides linked to RNA; for example, PROfusion technology, Phylos, Inc. See, for example, Roberts and Szostak (1997), Proc. Natl. Acad. Sci. USA, 94: 12297-303. Hereinafter, this and related methods are collectively referred to as xe2x80x9cRNA-peptide screening.xe2x80x9d Chemically derived peptide libraries have been developed in which peptides are immobilized on stable, non-biological materials, such as polyethylene rods or solvent-permeable resins. Another chemically derived peptide library uses photolithography to scan peptides immobilized on glass slides. Hereinafter, these and related methods are collectively referred to as xe2x80x9cchemical-peptide screening.xe2x80x9d Chemical-peptide screening may be advantageous in that it allows use of D-amino acids and other unnatural analogues, as well as non-peptide elements. Both biological and chemical methods are reviewed in Wells and Lowman (1992), Curr. Opin. Biotechnol. 3: 355-62. Conceptually, one may discover peptide mimetics of any protein using phage display, RNA-peptide screening, and the other methods mentioned above.
The present invention concerns therapeutic agents that modulate the activity of PTH and PTHrP. In accordance with the present invention, modulators of PTH and PTHrP comprise:
a) a PTH/PTHrP modulating domain, preferably the amino acid sequence of PTH/PTHrP modulating domains of PTH and/or PTHrP, or sequences derived therefrom by phage display, RNA-peptide screening, or the other techniques mentioned above; and
b) a vehicle, such as a polymer (e.g., PEG or dextran) or an Fc domain, which is preferred;
wherein the vehicle is covalently attached to the carboxyl terminus of the PTH/PTHrP modulating domain. The preferred vehicle is an Fc domain, and the preferred Fc domain is an IgG Fc domain. Preferred PTH/PTHrP modulating domains comprise the PTH and PTHrP-derived amino acid sequences described hereinafter. Other PTH/PTHrP modulating domains can be generated by phage display, RNA-peptide screening and the other techniques mentioned herein. Such peptides typically will be antagonists of both PTH and PTHrP, although such techniques can be used to generate peptide sequences that serve as selective inhibitors (e.g., inhibitors of PTH but not PTHrP).
Further in accordance with the present invention is a process for making PTH and PTHrP modulators, which comprises:
a) selecting at least one peptide that binds to the PTH-1 or PTH-2 receptor; and
b) covalently linking said peptide to a vehicle.
The preferred vehicle is an Fc domain. Step (a) is preferably carried out by selection from the peptide sequences in Tables 1 and 2 hereinafter or from phage display, RNA-peptide screening, or the other techniques mentioned herein.
The compounds of this invention may be prepared by standard synthetic methods, recombinant DNA techniques, or any other methods of preparing peptides and fusion proteins. Compounds of this invention that encompass non-peptide portions may be synthesized by standard organic chemistry reactions, in addition to standard peptide chemistry reactions when applicable.
The primary use contemplated for the compounds of this invention is as therapeutic or prophylactic agents. The vehicle-linked peptide may have activity comparable toxe2x80x94or even greater thanxe2x80x94the natural ligand mimicked by the peptide.
The compounds of this invention may be used for therapeutic or prophylactic purposes by formulating them with appropriate pharmaceutical carrier materials and administering an effective amount to a patient, such as a human (or other mammal) in need thereof. Other related aspects are also included in the instant invention.
Of particular interest in the present invention are molecules comprising PTH/PTHRP modulating domains having a shortened PTH C-terminal sequence, such as PTH-(1-28) or (1-34). The prior art shows no anabolic studies using sustained duration delivery of such C-terminally truncated PTH fragments. Although the art does not suggest it, molecules comprising smaller fragments such as PTH-(1-30)-Fc can be anabolic on their own. Despite their weak agonism towards PLC (see Background of the Invention), hPTH-(1-30) is nearly as effective at CAMP stimulation as is hPTH-(1-34). While not wanting to be constrained by theory, the inventors note that the anabolic properties of PTH fragments may be selectively related to their CAMP activation, rather than PLC activation, so that PTH fragments with reduced receptor affinity will have a favorable anabolic profile. It is possible that continuous exposure to truncated PTH fragments would have a different, and more favorable effect on bone compared to continuous exposure to PTH-(1-34) or PTH-(1-84) that has been demonstrated in humans by Fraher et al. (1999).